ALBERT

Description: Dumortierite, ca . (Al,)Al 6 (BO 3 )Si 3 O 13 (O,OH) 2 , has a complex and unusual crystal structure that is shared by several minerals and an increasing number of synthetic materials, which we group together as dumortierite-like materials (DLMs). Dumortierite has a strongly pseudo-hexagonal orthorhombic structure based on rod-like double-chains of Al octahedra in a framework based on the {6·4·3·4} semi-regular planar tiling. The large hexagonal channels contain chains of face-sharing Al octahedra attached to the framework by rings of SiO 4 tetrahedra. Smaller triangular channels contain planar BO 3 groups, while in other materials the triangular channels are occupied by tetrahedral or pyramidal groups. Holtite and magnesiodumortierite are isostructural with dumortierite, while ellenbergerite, phosphoellenbergerite, ekatite, and the synthetic materials have very similar structures to dumortierite, but with hexagonal symmetry. These minerals and materials differ from one another in the identity of the metal cations occupying the framework and face-sharing chains, and in the tetrahedral, pyramidal, or triangular groups within the hexagonal and triangular channels. We write the following structure-generating function, which describes both topological and stoichiometric properties of DLMs: \[ \mathrm{X}{({\mathrm{M}}_{2}\mathrm{\Phi })}_{3}{({\mathrm{T}}^{h}{{\mathrm{\Theta }}^{h}}_{3}{\mathrm{\theta }}^{h})}_{3}({\mathrm{T}}^{t}{{\mathrm{\Theta }}^{t}}_{3}{\mathrm{\theta }}^{t}) \] where X is the face-sharing hexagonal channel octahedrally coordinated site, M is the framework octahedrally coordinated site, T h and T t are three- to four-coordinate cation sites in the hexagonal and triangular channels, is a four-coordinate anion site, h and t are three-coordinate basal anion sites, and h and t are one- to three-coordinate apical anion sites. We discuss various structural and crystal chemical properties of DLMs, illustrating their commonalities as variations on the theme described by the structure-generating function. We also discuss three closely related structures which involve modifications of different geometrical or topological aspects of the DLM structure: the structure of lyonsite and its synthetic analogues, the structure of satterlyite and holtedahlite, and the structure of the AB polytype of the cancrinite group of feldspathoids.

Description: Dumortierite samples from two pegmatite localities in Antarctica and one each in Germany and Russia show a range of As and Sb compositions (As + Sb + minor Bi = 0.001–0.212 apfu) and low Ta + Nb + Ti (0.001–0.079 apfu). Single-crystal diffraction data obtained from crystals from each sample refined to R 1 = 0.0161–0.0285, the latter value for a twinned crystal. Initial refinements of three of the four crystals showed considerable electron density at the Sb1 and Sb2 sites; however, the atoms at these sites are also highly anisotropic, and consequently the sites were split into distinct As1, Sb1, As2, and Sb2 positions. Such distinct As and Sb sites are not seen in the isostructural mineral holtite, which contains considerably more As and Sb (and Ta, Nb, and Ti). Initial refinements also showed that in all four crystals, the atom at the All site, with occupancies of 0.81–0.88, is highly anisotropic with most of the positional displacement in the a direction. The Al1 site was then split into Al1a, Al1, and Al1b positions, whose occupancies refined to All 〉 Al1a 〉 Al1b. The unequal occupancy of Al1a, Al1, and Al1b suggests that the hexagonal channel contains a disordered mix of face-sharing octahedron dimers, trimers and longer units separated by vacancies. A plot of Si + P apfu versus As + Sb + Bi apfu for 340 dumortierite and 627 holtite compositions shows no gap between the two minerals. Although there is a pronounced gap in terms of As and Sb occupancy, it separates dumortierite and Sb-poor holtite from Sb-bearing holtite. The continuum of compositions between dumortierite and holtite, and the discovery of very (As, Sb)-rich, (Ta, Nb)-poor compositions, suggest that the distinction between what has been called dumortierite and what has been called holtite should be reconsidered.

Description: Meierite, ideally Ba 44 Si 66 Al 30 O 192 Cl 25 (OH) 33 , is a new mineral from the Gun claim, just south of the Itsi Range, Yukon, Canada. Meierite occurs as equant grains up to 200 μm across, enclosed within large gillespite crystals. The mineral is transparent, has a vitreous luster, and is non-fluorescent. It has a white streak and Mohs hardness of approximately 51/2. It is brittle with no observed cleavage. The calculated density based upon the chemical formula and single-crystal unit-cell dimension is 3.50 g/cm 3 . The mineral is optically isotropic ( n = 1.598) Electron-microprobe composition (average of 11): SiO 2 28.30, P 2 O 5 1.61, Al 2 O 3 11.75, TiO 2 0.05, FeO 0.27, CaO 0.21, BaO 47.61, Na 2 O 0.15, K 2 O 0.21, Cl 6.64, and a total of 95.29 wt.%. The empirical formula (based on 192 framework O apfu ) and charge balance considerations is: Ba 41.1 Ca 0.5 Fe 0.5 Na 0.7 K 0.6 Si 62.5 Al 30.5 P 3.0 O 192 Cl 24.8 ·33.4(OH). It is possible that additional H 2 O molecules are located within the cavities in the structure. Meierite is cubic, Im 3 m , a 18.5502(4) Å, V 6383.3(2) Å 3 , and Z = 1. The 10 most intense lines in the X-ray powder diffraction pattern are [ d obs in Å( I )( hkl )]: 4.39(70)(411), 4.16(26)(420), 3.798(25)(422), 3.288(34)(440), 3.189(100)(433), 3.016(72)(611), 2.803(42)(622), 2.629(31)(710), 2.323(46)(800), and 2.287(59)(741). The crystal structure ( R = 4.1% for 1393 F o 〉 4 F ) is a three dimensional framework of silicon-, aluminium-, and phosphorous-containing tetrahedra that create an open framework consisting of a large cubo-octahedral cavity connected by channels composed of double eight-membered rings and double six-membered rings. The aluminosilicate framework is isostructural with that observed for silicate framework type ZK-5 (KFI). The mineral is named in honor of Walter M. Meier (1926–2009), a pioneer in zeolite research.